The objective of this research proposal is to develop a procedure to estimate local and global greenwater loads at the point of contact between extreme wave crests and offshore structures. Through the combined efforts of laboratory measurements and numerical simulation, the result will allow designers to avoid or minimize the impact of greenwater on new floating structures through design, and help the industry and regulators to develop associated design guidance.

This research is a continued effort after the successful formulation of greenwater over a 2D platform through laboratory measurement, and a continuation on the development of a 3D computational fluid dynamics (CFD) code on the greenwater simulation. The prior study has shown that the traditional prediction method often used in design, i.e., the dam breaking model, results in significant discrepancy between the model and the laboratory measurements. Since the more realistic 3D prediction model is not yet established, the continuation of the research is critical for the prediction of greenwater and subsequently its mitigation. The project will consider 3D structure geometries such as TLP's, spars, and ship-shaped FPSO's.

In the experimental approach, the in-house developed bubble image velocimetry (BIV) method that is to be used in the present study to measure the velocity field in the highly turbulent, multi-phased greenwater flow has been further refined. The method is based on correlating the acquired images in the laboratory for greenwater velocity determination.  The in-house developed image processing software used in BIV has been modified and improved for faster velocity processing.  Part of the original code has been converted to Fortran from MATLAB.  The BIV post-processing software, also developed in house, has been improved to deal with the highly chaotic flow for better detecting stray velocity vectors in BIV results and replacing the vectors using the Kriging method.  The improved software is ready to be employed.  In addition, the void fraction measurement using fiber a optic reflectometer (FOR) on a 2D structure has been analyzed to obtain the void fraction distribution, overtopping flow rate, overtopping volume, and momentum flux of green water on the deck for force prediction.

In the numerical approach, OTRC  developed a Finite-Analytic Navier-Stokes (FANS) method in conjunction with an interface-preserving level-set method for the simulation of greenwater on two- and three-dimensional offshore structures.  In numerical wave tank simulations, open boundaries enclosing the fluid domain are artificial and essentially arbitrary.  In order to prevent unphysical wave reflections from the computational domain boundaries, a damping function was implemented on the downstream and sidewall boundaries to reduce the wave amplitude in the absorbing beach zone.  For long duration simulations, it is also necessary to prevent the reflected and diffracted waves from reaching the wavemaker boundary. In this study, concurrent computations were performed for the incident wave field (without structure) and the total wave field (with structure) simultaneously.  A second damping function was then implemented to suppress the differences between the total and the incident wave fields in front of the wavemaker.  This enables us to absorb the true wave reflection and diffraction from the offshore structures.

During the most recent reporting period ending June 2007, considerable effort has also been devoted to the generation of random waves and breaking waves.  A piston-type wavemaker was used successfully for the generation of random wave fields around a vertical circular cylinder in a numerical wave tank.  Breaking waves were also successfully generated in the numerical wave tank by the superposition of a long wave and a group of shorter waves.   Time-domain simulations are currently being performed for the impingement of the breaking waves on a two-dimensional rectangular platform.  The simulation results will be compared with the experimental data to provide a detailed validation of the level-set FANS method.  The method will then be employed for the simulation of breaking waves and greenwater effects on a three-dimensional platform.

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Last Updated: 03/06/2008, 12:33 PM